We have endeavoured to determine the fate of the adults and children presenting with bilateral optic neuritis who were reported by Hierons and Lyle (1959) and Meadows (1969). We have follow-up information from a variety of sources on 31 of the 34 adults for up to thirty-seven years, and on 17 of 19 children after up to thirty-two years. Three adult cases were excluded after review. The visual prognosis was variable in the adults but excellent in the children. None of the children has developed clinically definite multiple sclerosis. compared with 8 of 20 with sequential optic neuritis. We conclude that the risk of developing multiple sclerosis after simultaneous bilateral optic neurities in childhood is low; in adult life it is probably higher, but some patients even after more than two decades have no clinical evidence of the disseminated disease. Bilateral optic neuritis is aetiologically and prognostically diverse.